The present invention relates to a cathode ray tube (CRT), and particularly to a cathode ray tube having an electron gun capable of improving focus characteristics, correcting deflection defocusing and thereby providing a sufficient resolution over the entire phosphor screen and over the entire electron beam current region; a deflection-defocusing correcting member, a method of manufacturing thereof, and an image display system including the cathode ray tube.
A cathode ray tube such as a picture tube or a display tube includes at least an electron gun having a plurality of electrodes and a phosphor screen (a screen having a phosphor film, which is also referred to as "a phosphor film" or simply to "screen" hereinafter), and it also includes a deflection device for scanning an electron beam emitted from the electron gun over the phosphor screen.
A cathode ray tube of this type has an evacuated envelope comprised of a panel portion, a neck portion and a funnel portion connecting the panel and neck portions, and a deflection device mounted exteriorly around the evacuated envelope. A shadow mask is disposed a short distance from the phosphor screen inside the panel portion to control an electron beam to impinge upon a phosphor dot of intended color.
With such a cathode ray tube, there have been known the following techniques for obtaining a desired reproduced image over the entire phosphor screen from the center to the peripheral portions.
In such a cathode ray tube, deflection defocusing occurs due to variations in a distance between an electron gun and a phosphor screen with deflection angle of an electron beam. An electron beam spot is almost circular at the center of the phosphor screen without deflection defocusing. But at edges w and corners halo occurs due to deflection defocusing and blurs the electron beam spot, resulting in deterioration of resolution.
Japanese Patent Publication No. Hei 4-52586 discloses an electron gun emitting three inline electron beams in which a pair of parallel flat electrodes are disposed on the bottom face of a shield cup in such a manner as to be positioned above and below paths of the three electron beams in parallel to the inline direction and to extend toward a main lens.
U.S. Pat. No. 4,086,513 and its corresponding Japanese Patent Publication No. Sho 60-7345 disclose an electron gun emitting three inline electron beams in which a pair of parallel flat electrodes are disposed above and below paths of the three electron beams in parallel to the inline direction in such a manner as to extend from one of facing ends of one of a pair of main-lens-forming electrodes toward a phosphor screen, thereby shaping the electron beams before the electron beams enter a deflection magnetic field.
Japanese Patent Laid-open No. Sho 51-61766 discloses an electron gun in which an electrostatic quadrupole lens is formed between two electrodes and the strength of the electrostatic quadrupole lens is made to vary dynamically with the deflection of an electron beam, thereby achieving uniformity of an image over the entire screen.
Japanese Patent Publication No. Sho 53-18866 discloses an electron gun in which an astigmatic lens is provided in a region between a second grid electrode and a third grid electrode forming a prefocus lens.
U.S. Pat. No. 3,952,224 and its corresponding Japanese Patent Laid-open No. Sho 51-64368 discloses an electron gun emitting three inline electron beams in which an electron beam aperture of each of first and second grid electrodes is formed in an elliptic shape, and the degree of ellipticity of the aperture is made to differ for each beam path or the degree of ellipticity of the electron beam aperture of the center electron gun is made smaller than that of the side electron gun.
Japanese Patent Laid-open No. Sho 60-81736 discloses an electron gun emitting three inline electron beams in which a slit recess provided in a third grid electrode on the cathode side forms a non-axially-symmetrical lens, and an electron beam is made to impinge on the phosphor screen through at least one non-axially-symmetrical lens in which the axial depth of the slit recess is larger for the center beam than for the side beam.
Japanese Patent Laid-open No. Sho 54-139372 discloses a color cathode ray tube having an electron gun emitting three inline electron beams in which a soft magnetic material is disposed in fringe portions of the deflection magnetic field to form a pincushion-shaped magnetic field for deflecting the electron beams in the direction perpendicular to the inline direction of each electron beam, thereby suppressing a halo caused by the deflection magnetic field in the direction perpendicular to the inline direction.
FIG. 46 is a partially cut-away side view of an example of an electron gun for a cathode ray tube. Reference character K denotes a cathode, reference numeral 1 denotes a first grid electrode (G1), 2 a second grid electrode (G2), 3 a third grid electrode (G3), 4 a fourth grid electrode (G4), 5 a fifth grid electrode (G5), 6 a sixth grid electrode (G6), 30 a shield cup, and 38 a main lens. The electron gun is composed of the cathode, the first grid electrode 1, the second grid electrode 2, the third grid electrode 3, the fourth grid electrode 4, the fifth grid electrode 5 and the sixth grid electrode 6 arranged in the order named. The fifth grid electrode 5 is composed of two electrodes 51 and 52.
In FIG. 46, the length of different electrodes or the diameter of different electron beam apertures provide different effects of electric fields on the electron beam. For example, the shape of the electron beam aperture in the first grid electrode 1 close to the cathode 1 exerts an influence on the shape of the electron beam spot in a small-current region, while that in the second grid electrode 2 controls the shape of the electron beam spot in small- to large-current regions. In a main lens 38 formed between the fifth grid electrode 5 and the sixth grid electrode 6 supplied with an anode voltage, the shapes of the electron beam apertures in the fifth and the sixth grid electrodes 5 and 6 constituting the main lens exert an influence upon the shape of the electron beam in a large-current region, while they exert less influence on the shape of the electron beam in a small-current region compared with that in the large-current region.
The axial length of the fourth grid electrode 4 in the above-mentioned electron gun controls the magnitude of the optimum focus voltage and has a great influence upon a difference in optimum focus voltages between small-current and large-current operations, while the axial length of the fifth grid electrode 5 has markedly less influence compared with that of the fourth grid electrode 4.
For optimization of respective characteristics of the electron beam, the dimensions of a particular electrode most effective on the desired characteristics needs to be optimized.
When the aperture pitch in a direction perpendicular to the electron beam scanning line in a shadow mask is decreased, or the density of the scanning lines is increased, in order to enhance the resolution in a direction perpendicular to the scanning line, the scanning lines interferes with the periodic structures of the shadow mask and the contrast of resultant moire has to be suppressed. The prior art could not solve these problems.
FIGS. 47A and 47B are schematic views, each showing an essential portion of an electron gun, for comparing the two structures of the electron guns depending on the manner of supplying the focus voltage; wherein FIG. 47A shows a fixed-focus-voltage type electron gun; and FIG. 47B shows a dynamic-focus-voltage type electron gun.
The configuration of the electron gun of the fixed-focus-voltage type shown in FIG. 47A is the same as that shown in FIG. 46, and therefore, parts corresponding to those in FIG. 46 are indicated by the same characters.
In the electron gun of the fixed-focus-voltage type shown in FIG. 47A, a focus voltage Vf1 having the same potential is applied to the electrodes 51 and 52 forming the fifth grid electrode 5.